Applicant claims, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Feb. 26, 1999 of a German patent application, copy attached, Ser. No. 199 08 328.2, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The invention relates to an optical position measuring system having at least one scale and a scanning unit, which is movable relative thereto, and includes a light source, a reflector element, as well as at least one detector element, wherein the light beams emitted by the light source are deflected several times by the reflector element in the direction of the course of propagation and impinge several times on the at least one detector element, on which scanning signals, which are modulated as a function of displacement, can be detected.
2. Discussion of Related Art
A position measuring system in accordance with the species is known from WO 89/05964. An optical position measuring system, which is based on an interferential three-grating incremental transducer principle is proposed in this document; FIG. 1 shows the basic design of this position measuring system. It includes a scale M with an incremental measuring graduation MT, as well as a scanning unit A, which can be moved relatively thereto in the measuring direction x. A light source L, a reflector element R, as well as several detector elements D and, if required, further signal processing elements, are essentially arranged in the scanning unit. In the course of the beam propagation, the light beams emitted by the light source L impinge several times on the measuring graduation MT and the reflector element R before interfering partial light beams are deflected in the direction toward the detector elements D following the last reflection. In case of a relative motion of the measuring graduation MT in relation to the scanning unit A, periodically modulated incremental scanning signals are present at the detector elements D, which can be evaluated in a known manner. The resulting signal period SP of the scanning signals is SP=TP/4 in this position measuring system, where TP identifies the graduation period of the measuring graduation MT.
The described design of this optical interferential position measuring system in accordance with the so-called three- grating incremental transducer principle now offers particular advantages as far as the insensitivity to twisting of the scanning unit A in respect to the scale M is concerned. However, it is considered to be disadvantageous that a signal multiplication by only a factor of four results; moreover, the resulting signal modulation on the detector side is not sufficiently large. The reason for the latter lies in that there is the absolute requirement of designing the measuring graduation MT as a phase grating with a phase depth of xcex94xcfx86=xcex/2, which results in the appearance of a zeroth order of diffraction after the light beams have impinged on the measuring graduation MT. In turn, the zeroth order of diffraction acts as a constant light level for the detected scanning signals, which results in an undesired reduction of the signal modulation in the end.
Further optical position measuring systems designed as interferential three-grating incremental transducers are known from EP 0 163 362 B1, as well as from DE 24 31 551.
In this connection, FIG. 2 shows the extended beam path, as well as various important optical values, in accordance with the system of EP 0 163 362 B1. Furthermore, the position measuring system in the document mentioned has been extensively described in the publication of A. Spiess with the title xe2x80x9cLxc3xa4ngen in der Ultra- prxc3xa4zisionstechnik messenxe2x80x9d [Measuring Length by Means of Ultra- Precision Technology] in Feinwerktechnik and Me technik 98 [Mechanical Precision Technology and Measuring Technology], No. 10, pp. 406 to 410.
The light beams coming from a light source, not represented, are split at a first scanning graduation AG into the zeroth, as well as the +/xe2x88x92 first order of diffraction. The diffracted partial light beams thereafter reach the measuring graduation M, where another diffraction takes place before, following another passage through the second scanning graduation AGxe2x80x2, the split partial light beams come to interference. The scanning graduations AG, AGxe2x80x2, as well as the measuring graduation M are here respectively designed as phase gratings. The periodically modulated scanning signals resulting in case of a relative motion of the graduations AG and M are detected on downstream-connected detector elements, not represented.
A ratio of fringe widths w and gap widths f, which differs from 1:1, is provided on the side of the phase grating scanning grating AG. The fringe widths w and gap widths f on the side of the phase grating measuring graduation M have been selected to be identical; the phase depth xcex94xcfx86 of the measuring graduation M is xcex/2, which in the end assures a satisfactory modulation degree of the scanning signals on the detector side. Otherwise, the graduation periods d of all graduations M, AG, AGxe2x80x2 involved have been selected to be identical.
The basic beam path of the system from DE 24 31 551 is sketched in FIG. 3. Reference is furthermore made in connection with this variation of an interferential three-grating incremental transducer to Ch. 4 of a dissertation of J. Wilhelm xe2x80x9cDreigitter-schrittgeberxe2x80x9d [Three-grating Incremental Transducer] of the TU [Technical University] Hannover, 1978.
All gratings AG, AGxe2x80x2, M used in the optical position measuring system proposed therein are designed as phase gratings with a phase depth xcex94xcfx86=xcex/2, which again assures high efficiency. Here, the graduation periods dAG, dAGxe2x80x2, of the scanning graduation(s) AG, AGxe2x80x2 have been selected to be twice as large as the graduation period dM of the measuring graduation M. All gratings AG, AGxe2x80x2, M used respectively have fringe and gap widths of the same size. Because of the phase depth xcex94xcfx86=xcex/2 in all graduations, this system assures the suppression of the zeroth order of diffraction, and therefore provides a good degree of modulation of the scanning system.
However, it must be noted as disadvantageous in connection with this, as well as with the previous discussed optical measuring system, that both systems are relatively sensitive to twisting and tilting of the scanning unit in respect to the scale, i.e. such possible twisting during the measuring operation results in incorrect measurements.
It is therefore an object of the present invention to disclose a position measuring system that is insensitive to a large extent to possible twisting of the scanning unit in respect to the scale. Moreover, satisfactory signal modulation, as well as the greatest possible signal multiplication, are desirable.
An optical position measuring system meeting these requirements includes a scale that has at least two partial measuring graduations that have different graduation periods and a scanning unit that is movable relative to the scale along a measuring direction. The scanning unit includes a light source that emits light beams, a reflector element and a detector element, wherein the light beams emitted by the light source are deflected several times by the reflector element in the direction of the course of propagation of the emitted light beams and impinge several times on said scale before they impinge on said detector element, on which scanning signals, which are modulated as a function of displacement, are detected.
The steps in accordance with the present invention now substantially guarantee that the desired insensitivity to twisting during measuring operations is assured, as well as a satisfactory degree of modulation of the resulting scanning signals. The signal multiplication, i.e. the ratio of the graduation period and the signal period of the scanning signals, which can be achieved by means of the optical position measuring system of the invention, is moreover also greater than with the discussed systems in accordance with the prior art.
Furthermore, a number of embodiment options exists within the scope of the present invention. Thus, linear, as well as rotary measuring systems can of course be realized on the basis of the present invention. In the same way, combinations of different measuring graduation variations can be used, for example systems consisting of incident light and transmitted light measuring graduations, etc. Furthermore, many possibilities exist for adapting the present invention to the most different measuring conditions.
Further advantages, as well as details of the optical position measuring system in accordance with the invention ensue from the subsequent description of several exemplary embodiments by means of the attached drawings.